You need to "reinsert" it manually. It's very well explained in this Howto: http://www.tldp.org/HOWTO/Software-RAID-HOWTO.html See the section 6.3 to know how to add the new drive to the array.
I tested this 3 days ago :)
There has to be a better way than adding manually. We ran RAID1 on 2.2 kernels for years, and anybody, could replace a drive And it would be added automagically...
Maybe adding "auto" to mdadm.conf and better describing the devices.
Gerald